Although Tehran finds it difficult to sell its own crude under sanctions, two of its ships picked up Syrian oil late in July following a smaller purchase earlier.

Iran has become Syria's main crude oil buyer, helping Bashar Assad's flailing government to circumvent Western sanctions, as Iranian tankers have returned for a third time since April, shipping industry sources said.

Although Tehran already finds it difficult to sell its own crude under sanctions, two of its ships picked up Syrian oil late in July following a smaller purchase at the start of the same month.

Asia is the likely destination for the Syrian oil, but the sources said it remained unclear where the previous cargoes ended up.

Iran sells its own crude to only a handful of buyers compared to its previous client list. China, India, Japan, Turkey have still been buying after overcoming shipping obstacles, and South Korea will probably resume next month.

The MT Tour and MT Amin, two tankers under the flag of Sierra Leone and owned by Islamic Republic of Iran Shipping Lines (IRISL), which is under United Nations sanctions, lifted around 140,000 tons each of Syrian Heavy crude from the Syrian port of Tartous in the last few days of July.

The Tour is now moving through the Suez Canal and heading to the Gulf of Oman, while the Amin is still in the Mediterranean, ship tracking data showed, and is heading to the Arab Gulf, according to a shipping source.

The European Union and the United States imposed oil sanctions against Syria last year in an effort to put pressure on Assad's administration by cutting vital oil product imports and revenues from oil sales.

Protests inspired by the Arab Spring movement, which began last year, have developed into civil war as Assad's crackdown on opposition continues.

Syria was still able to import oil products from Russia and cooking gas from Greece but tightening sanctions cut off those avenues in April.

The government now seems heavily reliant on Iran, itself under sanctions by the EU and U.S. over its nuclear ambitions. Syria is also Iran's top destination for arms sales, according to the UN.

Earlier this year, the Tour, laden with Syrian crude, was headed for Singapore but turned back and dropped anchor at the Bandar Abbas port in Iran.

At the beginning of July, the Amin loaded a combination cargo of Syrian Light and Heavy at Tartous and Banias and another Iranian tanker, the Alvan, discharged around 30,000 tons of Iranian diesel - or gasoil as it is known in the industry - in exchange for gasoline, which then returned to Iran at the port of Bandar Abbas.